1. Field of the Invention
The present invention relates to a pressure plate used when a liquid crystal panel is produced by pressurizing two (upper and lower) glass substrates containing a liquid crystal between them, as well as to a process for producing a liquid crystal panel by the use of said pressure plate.
2. Prior Art
As a press used for production of liquid crystal panels, there has hitherto been known, for example, an apparatus disclosed in Japanese Patent Application Kokai (Laid-Open) No. 301814/1992. With this apparatus, two (upper and lower) glass substrates each provided with a transparent electrode are arranged with a given gap provided between them; spacers and a liquid crystal are placed in the gap and the periphery of the resulting material is sealed; then, the material is pressurized at normal temperature by the use of a pair of pressure plates (a pressurizing plate and a pressure-receiving plate) to conduct bonding; the material after pressurization is heated to about 200.degree. C. to allow the liquid crystal to be uniformly present in the gap; thereby, a liquid crystal panel is produced.
The pressure plates used in such a process for liquid crystal production have been stainless steel plates of about 200 mm.times.250 mm, in many cases. As the liquid crystal panel has become larger in recent years, pressure plates of about 400 mm.times.350 mm or larger have become necessary; and investigations are being made to use, in addition to stainless steel plates, pressure plates made of alumina, crystallized glass, graphite, silicon carbide or the like.
Each of these conventional pressure plates for use in a press, however, has problems. For example, stainless steel plates have a large thermal expansion coefficient although they have a high electric conductivity; therefore, when heated to about 200.degree. C., they produce a strain and it is highly possible that; the liquid crystal panel obtained has a strain at the image surface. The pressure plates made of crystallized glass, graphite or the like have a low electric conductivity, are easily charged with static electricity and adsorb dust, producing a defective liquid crystal panel. The pressure plates made of pure silicon carbide have a large shrinkage ratio when fired although they have a high electric conductivity; therefore, they are greatly deformed by warpage when heated; this makes the produced liquid crystal panel rough at the center when the panel is a thick panel, and a longer time is required for the finish.